Messerschmitt Me.209-II

The Messerschmitt Me.209 was used also for a proposal made in 1943 to make a heavily modified version of the Bf.109. This proposed version was intended to compete against Focke-Wulf's Fw.190D-9 and Ta.152 fighters.
Just like its competitors, it would share most of its airframe with a consolidated model, which in this case it was the Bf.109G.
Unlickly for the design team, the proposed engine, the Daimler-Benz DB 603A was in short supply and therefore they were forced to use the Junker Jumo 213A engine. Even if the 35 litre engine displacement Jumo 213 had been designed on purpose to have as many of its engine access points as possible made to be like the 44,52 litre displacement DB 603 powerplant (which was Germany's largest-displacement inverted V12 aircraft engine), that changeover required some reconstruction of both the engine cowling and cooling system. The most noticeable one was the one made to the engine's air intake location, as the Jumo 213's supercharger was placed at the starboard side of the engine, as it was standard for all models of the earlier Jumo 211 inverted V, and the DB.603A had it placed at the portside, which was the standard side for every Daimler-Benz inverted V12 designs.
The Me.209 featured also a new tail section, wings, wide-track landing gear, taller tail and an annular radiator for the inline engine, giving this the engine a superficial resemblance to a radial engine and to the engine installation of a Focke-Wulf Fw.190D, which was powered by the same Jumo 213 engine model.
All those changes altered too much the original purpose of the aircraft which was to build a superior aircraft resembling the Bf.109G as much as possible.
The Me.209V-5 ,which coontrary to what the numberation could suggest was the first prototype, was armed with one 30mm Mk.108 motorkannone firing through the propeller hub and two 13mm MG 131 machine guns placed in the wing roots. The Me.209V-6 was the first one converted to use the Jumo 213 and was armed with a single 20mm MG.151/20 motorkannone gun and two 13mm MG 131 machine guns. There was a high-altittude variant, which also saw one prototype called Me.209H V1 with extended wings and DB 603 engine driving a four-bladed propeller.
The programme was abruptly cancelled when the Me.209V5 prototype flew for the first time in late 1943 and showed to be 50km/h (31mph) slower than the Fw.190D, which was already in service, and offered no improvement in handling characteristics. After its disappointing show, the Me.209 project was cancelled ending that way Messerschmitt's last attempt at building a high-performance piston-engined fighter.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Me_209_(1943)
2. Salamander Books - The Complete Book of Fighters